On 01/15/2017 07:35 AM, Paul Koning wrote: > Dijkstra's objection wasn't to linkage via a register -- after all, > his earlier machine (the Electrologica X1) does that too, or more > precisely via one of 16 low core locations. The problem with the > 1620 is that the link register is not readable -- it's an invisible > register. And that's pretty much unique; all the other machines use > a program-accessible register. So you can context switch the link > address, but not on the 1620.
More precisely, as I've noted, the P-counter is a write-only register--there is no way for a program to determine its contents. Why IBM chose never to remedy this situation in later revisions is a mystery. > The other obvious issue with the 1620 is that it has no interrupts, > so it's hard to see how you would do multiprogramming. If you wanted interrupts, you ordered the 1710 variant of the 1620; complete with RTC. But the 1710 was used mostly for process control. Akin to the 1130/1800 distinction. > And one other, more obscure, objection is that while it has paper > tape readers, it's incapable of reading paper tapes with arbitrary > data. It only accepts tapes punched with the specific character > coding it likes. I suspect that the 1620 was not alone in this. After all, it's a *decimal* machine, which means that alphamerically, it could represent 100 alphameric characters plus record mark, numeric blank and group mark. That's short of the 128 characters that a 7-level paper tape can represent. Dijkstra's objection to the record mark issue is that I/O on the 1620 is value-delimited--on alphameric output, the delimiting record mark is not punched. However, it's perfectly possible to punch a record mark using a Write Numeric instruction. (I/O on the 1620 could be numeric or alphameric; thus a two-digit value could be punched alphamerically as a single character or numerically as two. Dig around in the list of old decimal machines and I'm sure that this behavior wasn't unique. My own thoughts on the 1620 I/O was that it was unnecessarily complex in the interests of keeping it variable-length. AFAIK, you couldn't read just one column of a card on the 1130--you always got the whole shebang. --Chuck
